Imagine finding out that your brother, a man you trusted, admired, and looked up to, decided his best course of action was to turn on his family and begin destroying everything you had all fought to achieve. Imagine finding this out, attempting to warn your father of this treachery, and being deemed a traitor for it. Imagine watching your world burn, your people slaughtered, and your soldiers massacred as a result of your attempt to warn your father of your brother’s treachery. Imagine that the only escape from total annihilation of your men, and people, was to make a pact with a once hated enemy. Can you imagine the frustration you’d feel? What about the resentment? This is the case of the Primarch of Thousand Sons, Magnus the Red.
Magnus is one of the few remaining Primarchs, however he currently survives as Daemon Prince of Tzeentch, having turned to the ruinous powers of chaos during the destruction of his home world by his brother Leman Russ and his Space Marine Legion called the Space Wolves. However before those events, Magnus was known for his desire for knowledge, and his mastery of sorcery, being one of the most powerful psykers in the galaxy, second only to the God Emperor of Mankind. Like most Primarchs, Magnus was impressive in stature, and had skin that was red in color thus the name “Magnus the Red”. Like his fellow Primarchs, Magnus was stranded on a world away from Terra, where he was raised by physic scholars and leaders of the his new home, the world known as Prospero.
Prospero was settled mostly by human psykers, who chose to colonize this remote world due to its inaccessibility. The settlers of this world were seeking to escape persecution from “normal” human beings which looked down on their mutant psychic abilities. It is by these people that Magnus would be taught to harness his psychic abilities, which had been present since his gestation, eventually leading him to surpass all of his teachers. Magnus is unique among the Primarchs in that he was able to communicate with his father, even before being found. In fact it was through his psychic ability that Magnus and the God Emperor were able to keep in touch, so that when they did meet it was more like a reunion amongst old friends, rather than that of two men meeting for the first time.
Magnus was, and is, incredibly intelligent and is constantly attempting to gain further understanding in various areas of learning. He specially put a lot of time and effort in trying to understand the immaterium, and the “primordial creator” (Chaos) which ran the currents of the turbulent “Great Ocean” (what does from Prospero called the Warp). Among Magnus’ counselors was one of his wisest teachers, a man named Amon. Amon warned Magnus of the dangers of diving too deep into the Warp, a warning that Magnus often ignored. Once Magnus was reunited with his father, he went on decades long travels into the warp with the Emperor. The Emperor taught Magnus much more about the immaterium, however the Emperor also warned him about the dangers found within the immaterium.
Magnus did not heed the warnings of his teacher or his father, as he traveled into the warp for the purpose of finding a cure to rampant mutations faced by his chapter of Space Marines. Ultimately he did find the cure, in the form of a deal he made with an entity within the warp. His legion was saved from rampant mutation, at the expense of one of his eyes…law of equivalent exchanged I guess.
Magnus’ legion, named The Thousand Sons, developed powerful psychic abilities like their Primarch. These abilities proved useful in the pacification of worlds to be incorporated into the Imperium. The legion’s, and their Primarch’s, tendency for sorcery and psychic power drew the ire of Leman Russ and his Space Wolves. The Space Wolves saw the use of anything but physical combat as cowardice, and without honor. It is safe to say that despite being his brother, Leman Russ hated Magnus because he was a psyker. In turn the Space Wolves looked down upon their brothers of the Thousand Sons. In fact, psykers were often viewed with suspicion, and at times hatred, across the Imperium of man. This distrust towards psykers even permeated through out the Astartes Legions of the Imperial Fists, Death Guard, Raven Guard, and the Space Wolves.
Distrust of psykers within the Imperium lead to the Emperor banning the use of sorcery, or psychic powers, except for in some cases. Obviously Magnus was upset by this, and often tried to find justifications for the use of his abilities. This is where Magnus dun goofed though as he would ended up crossing the line in the eyes of the God Emperor of Mankind.
In the Warhammer 40k Universe there is a system of “tunnels” through the warp that is known as the “Webway”. The webway is used by the Eldar to traverse interstellar space, and the God Emperor had been working on a human webway to allow for safer faster than light travel. Magnus used this webway to get to Terra to warn his father of Horus’ betrayal/turn to chaos, after finding out of the betrayal through a vision that came to him when meditating. Magnus used his powers to traverse the webway leading to Terra only to find that a powerful spell created an impenetrable shield that prevented him from getting to his father. Technically since the use of sorcery was banned, the legal way to warn the Emperor was to use astrotelepathy, however this method was too slow thus why Magnus chose to project himself into the webway. As he found himself unable to break the seal that would allow him to speak to his father, Magnus ended up striking a deal with a warp entity which promised to give him the power to break the seal. Magnus had unknowingly made a deal with the Chaos God Tzeentch, which was the entity that had given him the power to break the seal, and in breaking the seal Magnus unknowingly released a daemonic invasion of Terra.
This Daemonic incursion was luckily contained withing the confines of the Emperor’s lab, where he was working towards the development of several secret projects. The Daemons were easily banished, but the line had been crossed, and the Emperor refused to listen to Magnus’ speak, instead moving to punish Magnus for having broken the law by using his psychic abilities. The Emperor was specially furious has Magnus’ actions had undone a lot of the work he had accomplished. As a result of his actions, The Emperor ordered the Space Wolves to bring in Magnus under custody to face punishment for his willful disobedience through the use of sorcery, as well as his destruction of the human webway.
The traitor Horus, intercepted the Space Wolves fleet and convinced Leman that bringing in Magnus would change nothing, and essentially ordered for the destruction of the Thousand Sons, and their home world of Prospero. Magnus knew of the approaching fleet of Space Wolves (supported by Sisters of Silence, Imperial Guard, and other Imperial Forces), but did nothing to prepare Prospero to defend itself, not even warning his own legion of the coming assault. This could have been an attempt to show that he meant no threat to the empire, and perhaps save his world, but ultimately it was a stupid move on his part as the assault was being lead by a brother that despised him driven by the goal to slaughter him.
The forces of Prospero were caught completely off guard due to Magnus’ lack of action. As a result of the lack of prepartion Prosperos forces took heavy casualties, and the world appeared as if though it would fall, this event came to be known as “The Fall of Prospero” or “The Burning of Prospero”. Magnus and Leman would meet on the battlefield, engaging in a ferocious battle between the brothers. Although Magnus was easily able to hold his own, Leman was able to blind Magnus in his one eye. As Magnus was blinded, Leman lifted him over his head and dropped him over his own knee to break his back. As defeat appeared to immanent, Tzeentch appeared before Magnus and offered to save his world, his legion, him, and his collection of knowledge. In order to save his people, himself, and his vast collection of knowledge, Magnus made a deal with Tzeentch. Through this deal Magnus sealed his fate as a traitor, having now turned to Chaos.
Chaos is not to be trusted. Under chaotic rule the legion were subject to horrible mutations that left many Astartes as mindless chaos spawn. The legion’s numbers began to dwindle, and once again their existence was not secure. In search of a cure for the legions’ rampant mutations, a council of the legion’s sorcerers cast a spell which ended up backfiring. The spell ended the mutations, but at the cost of the lives of non-psyker marines which instead became thralls whose bodies were destroyed, and souls were trapped in their armors. The outcome of this spell infuriated Magnus and he swore to destroy the council, a council made up of men that once served as his most trusted advisors. With the state of his legion, and the destruction of his planet, Magnus now sits upon the The Planet of Sorcerers in the Eye of Terror, plotting his revenge on the Empire he believes betrayed him. Honestly, Upon reading the entirety of Magnus’ story it is clear that his turn to chaos was a response to Imperial aggression, and Magnus really did nothing wrong.